Fixes #11855

Fix sliver padding assertion failure that sometimes happens during startup because the app is temporarily built with zero size

This patch adds a notification to SliverChildDelegate that says which
children are alive after each layout. The delegate can use this
information to optimize it's underlying model of the children (e.g., by
discarding models for children that are far outside the live range).

Fixes #9045

Previously they tried to compute an integer target end index, but
integers can't represent infinity. Now we use null to represent
infinity.

Also, fix some similar issues with grids.

Fixes #8398

Rename all the "2" classes related to scrolling to remove the "2". Now that the
old scrolling code is gone, we don't need to use the suffix.

We weren't triggering a relayout when the number of children changed.

Fixes #8001

This prepares us for a CustomScrollView that takes slivers.

This patch makes a number of changes to how you can configure a
Scrollable2:

 - The ScrollPhysics is now responsible for creating the ScrollPosition.
   You can override the ScrollPhysics by supplying a `physics` argument
   to `Scrollable`, and the new physics you supply will be applied to
   the default physics inherited from the ScrollBehavior.

 - This patch removes the ScrollPosition/AbsoluteScrollPosition split as
   all clients were operating in pixels anyway and the split made the
   code very difficult to follow.

 - ScrollPosition no longer depends directly on Scrollable2State.
   Instead, it depends on an abstract interface that Scrollable2State
   implements. This change has two benefits:

    a) It removes the circular dependency between ScrollPosition and
       Scrollable2State, which lets us split the code for these classes
       (and several other classes that got wrapped up in that cycle) into
       separate libraries for easier maintenance.

    b) ScrollPosition is no longer bound to Scrollable2, which means you
       could use the behavior machinery to drive other sorts of widgets.
       For example, we could use it to drive Scrollabe1 if we wanted.

SliverFill fills the remaining space in the viewport with each box
child. We'll use this sliver as a building block for pageable lists.

Add SliverList

A SliverList is a linear layout of box children in a viewport that all
have a common, fixed extent along the scroll axis. The layout is similar
to a SliverBlock but more efficient.